1. Field of the Invention
The present invention relates to a method of manufacturing hydrophilic carbon nanotubes.
2. Description of the Related Art
A carbon nanotube is a single tubular molecule where a large number of carbon atoms are continuously bonded in such a state that six-membered rings are formed by sp2 bonding and possesses a structure of nanometer order. The end part of the carbon nanotube is closed with forming a hemispherical structure by incorporating five-membered rings in part. Moreover, the above carbon nanotubes form single-layered tubes, or tubes of various sizes are sometimes layered in a nested form to form multi-layered tubes.
The above carbon nanotubes are expected to have application to various uses such as one-dimensional wires, catalysts, cold-cathode elements, and hydrogen storage substances owing to mechanical and structural properties derived from its specific structure. However, the carbon nanotubes have a structure where a large number of carbon atoms are continuously bonded and hence is low in chemical activity and poor in wetting ability toward water and various organic solvents. Furthermore, the carbon nanotubes have characteristics to aggregate itself by mutual intermolecular force. As a result, the carbon nanotubes have a problem that it is difficult to disperse it in water or various organic solvents.
For solving the above problem, there is known a technology that chemical modification is applied to the carbon nanotubes to impart hydrophilicity to the surface thereof.
As the process for applying the chemical modification, there is a process of opening the end part(s) of the carbon nanotubes by immersing the carbon nanotubes in a mixed acid of sulfuric acid and nitric acid and irradiating the same with an ultrasonic wave to effect oxidation treatment. In the process, it is considered that a carboxyl group is bonded to the opened end(s) of the carbon nanotubes (Smalley et al., SCIENCE, vol. 280, 22 May 1988, p. 1253-1256).
As an alternative process, there is known a process of bringing the carbon nanotubes into contact with a strong oxidizing agent such as nitric acid or sulfuric acid for a sufficient time to oxidize the surface and subsequently bringing the oxidized surface of the carbon nanotubes into contact with a suitable reactant. By the above process, hydrophilic functional group(s) can be introduced into the surface of the carbon nanotubes (WO96/18059).
However, in the wet process using sulfuric acid, nitric acid, or the like as mentioned above, hydrophilicity can be imparted to the carbon nanotubes by bonding the hydrophilic functional group(s) thereto but there arises an inconvenience that the structure of the carbon nanotubes themselves are damaged by the acid. Moreover, in the above wet process, closest attention should be paid at operations owing to the use of sulfuric acid, nitric acid, or the like and also there may be a possible economical disadvantage in the case that sulfuric acid, nitric acid, or the like is used in a large amount.